Switching devices are used, for example, in motor vehicles in order to control various vehicle functions. For instance, a steering column module typically has integrated therein a switching device having a plurality of individual switches. The switches are for controlling the windshield wiper and washer assembly, the blinkers, the high-beam lights of the vehicle, etc.
FIG. 1 illustrates a block diagram of a switch assembly in accordance with embodiments of the present invention. The switch assembly includes a switching device having a plurality of switches (in this example, a first switch SE1 and a second switch SE2) for controlling respective functions. For instance, first and second switches SE1, SE2 are for controlling first and second vehicle functions respectively.
Each switch SE1, SE2 is formed by a respective resistance branch. The resistance branch forming first switch SE1 includes a measurement resistance M1 and a switchable one of a plurality of switching resistances S11, S12, S13. Switching resistances S11, S12, S13 have different nominal resistance values amongst one another. The switch position of first switch SE1 depends on which switching resistance S11, S12, S13 is selected. First switch SE1 has three potential switch positions respectively corresponding with the three switching resistances S11, S12, and S13. As shown in FIG. 1, in this example, switching resistance S11 is selected such that first switch SE1 is in the switch position corresponding to switching resistance S11. Measurement resistance M1 is connected at one end to ground potential Gnd and is connected at its other end to a constant supply voltage Vcc through the selected one of the switching resistances S11, S12, S13. As such, in this example, the other end of measurement resistance M1 is connected to supply voltage Vcc through switching resistance S11 as first switch SE1 is in the switch position corresponding to switching resistance S11. Thus, in this switch position, the total resistance in the resistance branch forming first switch SE1 is measuring resistance M1 and switching resistance S11.
Likewise, the resistance branch forming second switch SE2 includes a measurement resistance R2 and a switchable one of a plurality of switching elements S21, S22, S23, S24. Switching resistances S21, S22, S23, S24 have different nominal resistance values amongst one another. The switch position of second switch SE2 depends on which switching resistance S21, S22, S23, S24 is selected. Second switch SE2 has four potential switch positions respectively corresponding with the four switching resistances S21, S22, S23, S24. As shown in FIG. 1, in this example, switching resistance S23 is selected such that second switch SE2 is in the switch position corresponding to switching resistance S23. Measurement resistance M2 is connected at one end to ground potential Gnd and is connected at its other end to supply voltage Vcc through, in this example, switching resistance S23 as second switch SE2 is in the switch position corresponding to switching resistance S23. Thus, in this switch position, the total resistance in the resistance branch forming second switch SE2 is measuring resistance M2 and switching resistance S23.
As shown in FIG. 1, the resistance branches of switches SE1, SE2, including the respective switching resistances (S11; S23 in this example) according to the respective selected switch positions, are connected in parallel to one another through a common electrical conductor to supply voltage Vcc.
The switching assembly includes a processor μC having analog-to-digital converter (ADC) inputs. In this example, the processor includes three ADC inputs A/D 1, A/D 2, and A/D3.
In an operation for detecting the switch position of first switch SE1, the actual voltage drop UM1 across measuring resistance M1 is measured. The measured voltage drop UM1 is supplied to ADC input A/D 1 for evaluation by the processor Likewise, in an operation for detecting the switch position of second switch SE2, the actual voltage drop UM2 across measuring resistance M2 is measured and supplied to ADC input A/D 2 for the processor. For both operations, the actual voltage UVcc of supply voltage Vcc is measured and supplied to ADC input A/D 3 for the processor.
Measuring resistances M1, M2 and switching resistances S11, S12, S13; S21, S22, S23, S24 have known nominal resistance values. For instance, the nominal resistance values of switching resistances S11, S12, S13 are 1 k ohms, 10 k ohms, and 100 k ohms. Supply voltage Vcc has a known nominal voltage value such as, for instance, 5V. Expected fixed set-points for the voltage drops across measuring resistors M1, M2 are obtained based on the nominal values. The predetermined fixed set-points are respectively associated with the different switch positions of switches SE1, SE2, and thus for the measured voltages UM1, UM2.
In the example in which first switch SE1 is in the switch position in which switching resistance S11 is selected, the measured voltage drop UM1 occurring across measuring resistance M1 depends on the actual resistance values of measuring resistance M1 and switching resistance S11 and the actual voltage value of supply voltage Vcc. However, the voltage drop expected to across measuring resistance M1 depends on the nominal resistance values of measuring resistance M1 and switching resistance S11 and the nominal voltage value of supply voltage Vcc. Thus, if any of the actual values differ from the corresponding nominal values, then the measured voltage drop UM1 will differ from the expected voltage drop. The same analysis applies to any other one of switching resistances and to second switch SE2.
Deviations of the actual resistance values from the nominal design values arise due to tolerances of the components and characteristic impedances or contact resistances at switches and connectors. This situation affects the entire switching device and can thus give rise to deviations in the measured voltages UM1, UM2 from the predetermined fixed set-points. If the deviations attain the order of magnitude of the intervals between the measured voltages UM1, UM2 associated respectively with the different switch positions, then this can lead to erroneous switch position information and thus to defective operation of the components powered by the switching device.